User defined literal arguments are not constexpr?

Question

I\'m testing out user defined literals. I want to make _fac return the factorial of the number.

Having it call a constexpr function works,

Answer 1

I don't know if there's a better way in C++11 to do this than the current accepted answer, but with relaxed constexpr in C++14, you can just write "normal" code:

constexpr unsigned long long int operator "" _fac(unsigned long long int x) {
    unsigned long long int result = 1;
    for (; x >= 2; --x) {
        result *= x;
    }
    return result;
}

static_assert(5_fac == 120, "!");